Calibrating on Imagesetter Paper Technical Information Linotype-Hell
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1. procedure is consistent See your den The chart The chart shows a 0 00 density for a 0 dot This is because the densitometer has been zeroed to the white of the paper For the same reason the dmax of a solid is closer to 1 60 than to 1 70 Using the chart Formulas sitometer user manual for more information Pay particular attention to procedures for calibrating and zeroing your densitometer If your reflection densitometer can t read dot percent the chart to the right will help It takes into account some of the factors that are rela tively common to imagesetter paper dmin of around 07 dmax of around 1 7 screen ruling between 60 and 100 lines per inch and an esti mated reflective coefficient It will allow you to take a density value and convert it into dot per cent If for example you measure a 39 density you can see that this corresponds to a 50 dot This information can then be used in a calibra tion program to more accurately match your requested dot percent to the actual dot percent Accurately setting the dmax will assure that your dot percentages are as close as possible before calibration If the dmax is properly set the dot percent values should not be off by much more than several percent points Dot percentages will be more accurate at higher resolution settings Greater accuracy may be obtained by using a calibration program To use the chart measure each halftone area with the den2. screen rulings quality may suffer Achieving a reasonable dmax is the most important issue for quality paper output Calibrating dot percent can further improve the quality of halftones and tints But remember you should consider outputting to film if the screen ruling of your tints and halftones exceeds 100 lines per inch Please direct any questions or comments to Jim Hamilton Marketing Department Linotype Company 425 Oser Avenue Hauppauge NY 11788 516 434 2717 1991 Linotype Hell Company All rights reserved Linotronic is a registered trademark and Linotype Utility is a trademark of Linotype Hell AG and or its subsidiaries PostScript is a registered trademark of Adobe Systems Inc All other company and product names are trademarks or registered trademarks of their respective owners
3. 14 22 78 0 15 23 77 0 16 24 76 0 17 26 74 0 18 27 73 0 19 28 72 0 20 30 70 0 21 31 69 0 22 32 68 0 23 33 67 0 24 34 66 0 25 36 64 0 26 37 63 0 27 38 62 0 28 39 61 0 29 40 60 0 30 41 59 0 31 42 58 0 32 43 57 0 33 44 56 0 34 45 55 0 35 46 54 0 36 47 53 0 37 48 52 0 38 49 51 0 39 50 50 0 40 51 49 0 41 52 48 0 42 53 47 0 43 54 46 0 44 55 45 0 45 55 45 0 46 56 44 0 47 57 43 0 48 58 42 0 49 59 41 0 50 59 41 0 52 61 39 0 54 63 37 0 56 64 36 0 58 65 35 0 60 67 33 0 62 68 32 0 64 69 31 0 66 70 30 0 68 72 28 0 70 73 27 0 72 74 26 0 74 75 25 0 76 76 24 0 78 77 23 0 80 78 22 0 82 79 21 0 84 80 20 0 86 81 19 0 88 82 18 0 90 83 17 0 92 83 17 0 94 84 16 0 96 85 15 0 98 86 14 1 00 86 14 1 05 88 12 1 10 90 10 1 20 92 8 1 30 95 5 1 40 97 3 1 50 99 1 1 60 100 0 Yule Nielsen Murray Davies How to choose an n value Making a chart Conclusion Comments Part Number 3076 8 91 ters that convert density to dot percent usually use the Murray Davies formula to make their calculations dot area 100 x 1 10 BT 4 49 DS n where DT the density of the tint screened area DS the density of the solid area and n a constant for the light scattering characteristics of the paper Some sample n values coated paper 1 65 uncoated paper 2 70 dot area 100 x 1 10 PT 1 10 DS where n is assumed to be equal to one The Yule Nielsen formula allows you to tailor the formula to you
4. Linotype Hell Technical Calibrating on Information Imagesetter Paper One of the many choices you have to make when you output a job is whether you should output onto imagesetter paper or film If you output to film you can go directly to the platemaking process However if you want to create a mechanical also called a paste up you should choose paper RC or resin coated paper is the photographic paper that is used in PostScript imagesetters When tints or halftones are output on imagesetter paper special attention must be paid to assure that quality is maintained Why paper Imagesetter film can give better quality than imagesetter paper for the simple reason that imagesetter paper in a mechanical must be photographed to create films for platemaking This adds a step in the process that wouldn t be there if you had output to film However there are a variety of reasons why you might prefer to output to paper Price is one obvious reason Service bureaus often charge more for film than for paper But to truly compare the cost of film versus paper you must factor in any additional camera charges that result from having to shoot a mechani cal There are plenty of other reasons that are more important than cost e Itis easier to proofread and check a positive black amp white image e If copies need to be sent to different locations it may be easier and cheap er to output more than one on paper e It gives you the flexib
5. ility to cut and rework the design on a mechanical Some people are simply more comfortable with paper than with film because it is easier to read photocopy or photostat The limitations of paper When you make the decision to output paper instead of film it is important to understand some of the limitations of imagesetter paper e Adding an extra step into any process opens up room for error By having to shoot a mechanical you add some level of uncertainty e The consistency of any tint or halftone work is dependent on the quality of the work of the camera operator who shoots the mechanical e Shooting a mechanical that includes tints or halftones becomes more diffi cult as the screen ruling gets finer The task is easier for tints and halftones under 100 lines per inch Up to 133 lines per inch may be possible with a good camera and camera operator e For calibration you need to be able to measure dot percent Measuring dot percent on paper is not nearly as straightforward as measuring dot percent on film In fact many densitometers do not even give you a dot percent value when you measure on paper Therefore you have to convert a densi ty measurement into dot percent value This will be discussed in greater detail later on To understand some of the issues involved in calibration on paper we ll have to look at the nature of imagesetter paper including maximum density dmax and how dot percent is measured Maximum densi
6. nsitometer mea sures transparent materials like film See Figures 1 and 2 While most transmission densitometers can measure both density and dot percent many reflection densitometers measure only density Therefore to be able to cali brate on imagesetter paper you need one of two things A reflection densitometer that can read dot percent A chart that allows you to convert density to dot percent Reflection densitometers that read dot percent usually require a few extra steps You often need to measure the minimum density dmin i e the white of the paper and the dmax of the paper Using these figures it can calculate a dot percentage The nature of these calculations can provoke some con troversy among those involved in the study of densitometry because other factors can also play a role the reflective coefficient of the paper the screen ruling and the relative size of the halftone dot If you take into account more of these factors the calculations can become quite complex See the section on Formulas for more information on this subject There is one other way to attack this problem Shops that do camera work internally may prefer to shoot a mechanical and then measure the tints and halftones on the resulting film with a transmission densitometer One last point about densitometers The measurements you take may vary from one densitometer to the next Measurements may even vary from day to day Be sure that your measurement
7. r own condi tions From test results on imagesetter paper an n value of 1 7 gives results that correspond to reality But how can you determine your own n value One way to measure this variable is to work backwards using the formula Print out a blend from 0 to 100 or a step wedge with many values around 50 Under a magnifier identify the characteristic checkerboard shape of the 50 dot and measure the density value there it might be 51 Measure the dmax it might be 1 89 Be sure to normalize these values by subtracting the dmin from them The n value in this case would be approximately 2 Here are the calculations dot area 100 x 1 10 DT N 4 49 DS n 50 100 x 1 10 91 7 1 10 1 89 7 substituting 2 for n 100 x 1 56 1 11 100 x 44 89 49 4 3The characteristic checkerboard of the 50 halftone dot will not occur with certain halftone dot shapes line and ellipse for example Once you have an n value and a d max you can create a chart for your own conditions using a spread sheet However because screen ruling plays a role in this you may want to repeat this test for a number of screen rulings If you have decided to output to imagesetter paper remember that imaging to paper adds an extra step in the pre press process In the case of coarser screen rulings 65 to 85 lines per inch this is not crucial but be careful about going to paper for halftones higher than 100 lines per inch At these higher
8. rol and altering the density setting i e laser intensity on your imagesetter to get the dmax that you desire When you are working with imagesetter paper the characteristics are a little different than with imagesetter film After a certain point increasing the laser intensity does not make the dmax any darker However although the dmax doesn t darken increasing the laser intensity too high causes the fine detail to degrade One side effect of this is that halftone dots will start to enlarge This hinders any calibration efforts The easiest way to do get the proper dmax is to use the calibration portion of the Linotype Utility It allows you to print a test at a range of density settings When you have output and processed your range of tests measure the dmax produced Look for the point at which the dmax tops out The chart to the left shows a range of density settings from 120 to 720 in steps of 100 You can see that the dmax increases rapidly between between 120 and 320 but flattens out after that For this resolution a density setting between 220 and 320 would be appropriate These tests must be done for each resolution of the imagesetter that you intend to use Once the laser intensity setting is producing a reasonable dmax then you can start looking at the second step in calibration dot percent A reflection densitometer is the tool that is used to measure density on an opaque surface like paper Conversely a transmission de
9. sitometer and convert those values into dot percent Then pro ceed as you would normally during calibration Be sure that your densitometer is zeroed to the paper use the so called null setting which subtracts the paper dmin value from each reading Don t worry if the dmax of your page is a little higher than 1 60 it won t make much difference Note how little change there is in the last few steps of the chart There are other charts that have been published to convert density to dot percent for example DuPont has published one If you compare them to this chart you will see that the conver sions that they give are quite different This is because they have used different variables The formulas used to convert density to dot per cent are complex and will not be of interest to everyone However if you would like to develop a chart that is tailored for your own conditions here is the information that you will need There are two formulas that are used for con verting density values to dot percent both named after the people who developed them and supported their use There are arguments in favor of using either formula but it appears to me that Yule Nielsen is most appropriate for those using imagesetters However densitome Density Dot Neg Dot 0 00 0 100 0 01 2 98 0 02 3 97 0 03 5 95 0 04 7 93 0 05 8 92 0 06 10 90 0 07 11 89 0 08 13 87 0 09 14 86 0 10 16 84 0 11 17 83 0 12 19 81 0 13 20 80 0
10. ty Setting density Density Measured Setting Dmax 120 1 32 220 1 73 320 1 81 420 1 82 520 1 83 620 1 85 720 1 83 This test was run at 1270 dot per inch resolution Results will vary from imagesetter to imagesetter depending ona number of factors primarily film processor conditions The density setting ona Linotronic imagesetter con trols the intensity of the laser Densitometers Film Y Figure 1 A transmission densito meter measures the light that passes through a material Paper a 2 Figure 2 A reflection densitome ter measures the light that reflects off of a material Probably the most important thing you can do is to keep the dmax on paper within a reasonable range This depends on the type of film you use and may vary from 1 60 to 2 00 It is very difficult to get densities much higher than 2 00 on imagesetter paper However even though the density may not increase as you push your processing or laser intensity to get a higher den sity all of the tints halftones fine line work and serifs of small type may begin to degrade So it is very important to know the upper limit of your film and processing so that you don t go beyond it The first step in any calibration procedure should be to assure that you are getting a dmax that is appropriate for the imagesetter material and for your shop conditions This is best done by keeping your film processor under con t
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